How to tell which repositories provide which packages?

[Ralf Mardorf]

On Wed, 6 Jan 2021 12:41:39 -0500, Little Girl wrote:
>It looks like it must be an unsigned integer, but it can be ommitted
>and, if so, is assumed to be a zero:
>
>https://www.debian.org/doc/debian-policy/ch-controlfields.html#s-f-version

Thank you for doing this work. It's still not clear to me.

The epoch must start with a digit, but most packages don't use an
epoch at all. 

"The upstream_version may contain only alphanumerics [6] and the
characters . + - ~ (full stop, plus, hyphen, tilde) and should start
with a digit. If there is no debian_revision then hyphens are not
allowed."

"[6] Alphanumerics are A-Za-z0-9 only."

"should start" but IIUC they could start with a letter. A-Za-z seems to
exclude umlauts and other exotic signs. IIUC theoretically it could even
start with a full stop, plus, or tilde. IOW usage of "[:alnum:]" is
likely better than usage of "[:digit:]", probably no package version
starts with a full stop, plus, or tilde, but perhaps with "alpha",
"beta", "git", "af12de" hex numbers etc.